1. Field of the Invention
The present invention relates to optical recording medium driving devices, particularly, those for performing coarse adjustment of a spherical aberration correction value to enable tracking servo, and to spherical aberration adjustment methods.
2. Description of the Related Art
Data recording techniques that employ optical disc recording media (including magneto-optical discs), such as, for example, a compact disc (CD), a mini disc (MD), and a digital versatile disc (DVD), as recording media are known as techniques for recording and reproducing digital data. The optical disc recording media (also simply referred to as optical discs) is a general term referring to disc recording media on which signals are recorded as pits and marks and onto which laser light is irradiated to read the signals through changes in the reflected light.
Types of optical discs include playback-only types, such as, for example, a CD, a CD-ROM (read only memory), and a DVD-ROM, and user-data-recordable types, such as an MD, a CD-R (recordable), a CD-RW (rewritable), a DVD-R, a DVD-RW, DVD+RW, and a DVD-RAM. For the recordable types, a magneto-optical recording method, a phase change recording method, a dye film change recording method, and other recording methods are utilized to record data. The dye film change recording method, also referred to as a write-once recording method, allows data recording only once and does not allow rewriting. The dye film change recording method is therefore suitable for data storing purposes and the like. On the other hand, the magneto-optical recording method and the phase change recording method permit data rewriting, thereby finding many applications in recording of various contents including music, videos, games, and application programs.
Furthermore, recently developed high-density optical discs, called Blu-ray Discs®, offer an extremely high data storage capacity.
Regarding a structure of the discs, high-density optical discs, such as Blu-ray Discs, have a cover layer of approximately 0.1 mm in thickness of the discs. Data is reproduced from (or recorded on) the discs using a combination of a laser having a wavelength of 405 nm (so-called a blue laser) and an objective lens having numerical aperture (NA) of 0.85.
As is known, recording/reproducing apparatuses that record and reproduce data on optical discs perform a focus servo operation for controlling a focal position of a laser light onto a recording surface of discs, and a tracking servo operation for controlling the laser light to trace a track (a pit string or a groove) on discs.
It is known that an appropriate focus servo operation needs adding appropriate focus bias on a focus loop.
In particular, high-density optical discs require spherical aberration correction in order to cope with variations in the thickness of a cover layer and multiple recording layers. For example, an optical pickup including a spherical aberration correction mechanism employing an expander or a liquid crystal device has been developed.
Since recording/reproducing apparatuses, such as Blu-ray Disc recording/reproducing apparatuses, employing a high NA lens have a narrow margin for the focus bias and the spherical aberration, automatic adjustment of the focus bias and the spherical aberration is essential.
A spherical aberration correction value and focus bias are adjusted using some kind of evaluation value as an evaluation indicator. For example, a jitter value obtained in response to altering both the spherical aberration correction value and the focus bias can be employed as the evaluation value. The spherical aberration correction value and the focus bias are adjusted to minimize the jitter value.
For example, a case where a jitter value is used as an evaluation value at the time of adjustment assumes a state in which tracking servo can operate and the jitter value can be measured.
However, particularly regarding the spherical aberration correction value, the spherical aberration may deteriorate so much that the tracking servo does not operate when altering the spherical aberration correction value during adjustment in the above-described manner, which thus may prevent the appropriate adjustment operation from being performed.
Accordingly, prior to adjustment of a spherical aberration correction value and focus bias, the spherical aberration correction value is coarsely adjusted first in the related art.
More specifically, an amplitude value of a tracking error signal is obtained in response to altering a spherical aberration correction value with only focus servo being turned on, and the spherical aberration correction value is adjusted so that the amplitude value does not below a predetermined level. That is, the spherical aberration correction value is adjusted only as suitable as the tracking servo can operate.
By coarsely adjusting the spherical aberration correction value, the above-described fine adjustment of the spherical aberration correction value and the focus bias can be performed appropriately thereafter on the basis of the jitter value.
Japanese Unexamined Patent Application Publication No. 2004-95106 is an example of the related art.